Optical encoder

ABSTRACT

An optical encoder includes a code strip having a first side, a second side, a first track comprising indicia thereon, and a second track comprising indicia thereon. The code strip is moveable along a displacement path with respect to the optical encoder. A light source positioned on the first side of the code strip directs light toward the code strip. A first detector element is positioned on the second side of the code strip and is generally aligned with the first track of the code strip. A second detector element is positioned on the second side of the code strip and is generally aligned with the second track of the code strip. The second detector element is also positioned so that the second detector element is located a spaced distance along the displacement path from the first detector element.

BACKGROUND

Position and/or motion encoders provide a means for determining theposition and/or motion of moveable components. While a wide variety ofposition encoder systems have been developed and are being used, mostposition encoder systems can be placed into one of two categories:linear and rotary. As their respective names imply, linear encodersystems may be used to provide an indication of linear or straight-linemotion whereas rotary encoders may be used to provide an indication ofrotary motion.

Encoder systems of the type described above may be further characterizedas analog encoder systems or digital encoder systems. Analog encodersystems provide an analog output signal, such as a voltage or currentthat is related to the motion detected by the encoder. Analog encodersystems typically utilize a variable resistor or resistance element thatis operatively associated with the moveable element. The variableresistor converts the motion of the moveable component into the analogsignal.

Digital encoder systems provide a digital output signal that is relatedto the motion detected by the encoder. Most digital encoder systems areoptical in nature, although non-optical digital encoders are also known.An optical digital encoder typically utilizes a light source, adetector, and a code wheel or code strip. The code wheel or code stripis provided with markings or indicia thereon. The detector detects theindicia provided on the code wheel or code strip and produces a digitaloutput signal that is related to the position or movement of the codestrip with respect to the detector.

Digital encoders may provide a relative or absolute indication of therelative position of the code wheel or code strip. Generally speaking,relative encoders provide a single set of markings or indicia on thecode strip. Because the single set of markings is not unique to theparticular position of the code strip, relative encoder systems mustutilize a homing routine on start-up in order to derive the actualposition of the moveable component. Absolute position encoders typicallyrely on several sets of indicia on the code strip. The indicia are suchthat a unique signal is associated with each position of the code strip.Thus, such absolute position encoders can provide an indication of theabsolute position of the moveable element without the need to firstperform a homing routine.

SUMMARY OF THE INVENTION

An optical encoder according to one embodiment may comprise a code striphaving a first side, a second side, a first track comprising indiciathereon, and a second track comprising indicia thereon. The code stripis moveable along a displacement path with respect to the opticalencoder. A light source positioned on the first side of the code stripdirects light toward the code strip. A first detector element ispositioned on the second side of the code strip and is generally alignedwith the first track of the code strip. A second detector element ispositioned on the second side of the code strip and is generally alignedwith the second track of the code strip. The second detector element isalso positioned so that the second detector element is located a spaceddistance along the displacement path from the first detector element.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred exemplary embodiments of theinvention are shown in the drawings in which:

FIG. 1 is a perspective view of one embodiment of an optical encoder;

FIG. 2 is an exploded perspective view of the optical encoder of FIG. 1;

FIG. 3 is a side view in elevation of the optical encoder of FIG. 1;

FIG. 4 is a plan view of the light source assembly of the opticalencoder of FIG. 1;

FIG. 5 is an exploded perspective view of the light source assembly ofFIG. 4;

FIG. 6 is a plan view of the detector assembly of the optical encoder ofFIG. 1;

FIG. 7 a is a side view in elevation of a portion of the housing of theoptical encoder of FIG. 1;

FIG. 7 b is an enlarged side view of a portion of the housing moreclearly showing one of the bearing surfaces of FIG. 7; and

FIG. 8 is a plan view of the housing of FIG. 7 showing the positions ofthe bearing surfaces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One embodiment of an optical encoder 10 is shown in FIGS. 1 and 2 andcomprises a code strip 12 having a first side 14 and a second side 16.The code strip 12 comprises a plurality of tracks 17 with indicia 19provided thereon, such as a first track 18 of indicia 20 and a secondtrack 22 of indicia 24. As will be described in greater detail below,the code strip 12 may be provided with any number of tracks 17 ofindicia 19 thereon, depending on a number of factors, including, but notlimited to, the desired resolution and the range of motion to beencoded. The code strip 12 is moveable along a displacement path 26 withrespect to the optical encoder 10. In the embodiment shown and describedherein, a read head 28 of the optical encoder 10 is moveable withrespect to the code strip 12, which remains stationary. Alternatively,the code strip 12 could be moveable, with the read head 28 remainingstationary.

With reference now primarily to FIG. 2, the optical encoder 10 may alsobe provided with a light source assembly 30 and a detector assembly 32.The light source assembly 30 and detector assembly 32 may be mounted toa housing assembly 34. Thus, the read head 28 comprises the light sourceassembly 30, detector assembly 32, and the housing assembly 34. Thearrangement is such that the light source assembly 30 is positioned onthe first side 14 of the code strip 12, whereas the detector assembly 32is positioned on the second side 16 of the code strip 12. The detectorassembly 32 is also generally aligned with the light source assembly 30,so that light produced by the light source assembly 30 and passingthrough the code strip 12 can be detected by the detector assembly 32.

The light source assembly 30 is shown in FIGS. 4 and 5 and may comprisea plurality of light emitting elements 37, such as, for example, a firstlight emitting element 38 at a first location along the displacementpath 26 and a second light emitting element 40 at a second locationalong the displacement path 26. In the embodiment shown and describedherein, the light source assembly 30 is provided with additional lightemitting elements, as will be described in greater detail below. Each ofthe plurality of light emitting elements 37 is provided with acorresponding collimating lens 41, such as first collimating lens 42 andsecond collimating lens 44. The various light emitting elements 37 arepositioned in various locations to form the staggered spacingarrangement shown in FIG. 4. More specifically, in one embodiment, thefirst light emitting element 38 is positioned along a first lightemitting element axis 46, whereas the second light emitting element 40is positioned along a second light emitting element axis 48. The firstand second light emitting element axes 46 and 48 are separated by aspaced-distance 50 along the direction of the displacement path 26. Aswill be explained in greater detail below, this staggered spacing of thefirst and second light emitting elements 38 and 40 allows the overalllength 48 of the light source assembly 30 to be reduced compared to whatwould otherwise be the case if the individual light emitting elements 37(e.g., first light emitting element 38 and second light emitting element40) were aligned along a common axis.

The detector assembly 32 is best seen in FIG. 6 and may comprise aplurality of detector elements 51, such as a first detector element 52positioned at a first location along the displacement path 26 and asecond detector element 54 positioned at a second location along thedisplacement path 26. In the embodiment shown and described herein, thedetector assembly 32 is provided with additional detector elements 51,as will be explained in greater detail below. The first detector element52 is positioned along a first detector element axis 56, whereas thesecond detector element 54 is positioned along a second detector elementaxis 58. The first and second detector element axes 56 and 58 areseparated by a spaced-distance 60 along the direction of thedisplacement path 26. In one embodiment, the spaced-distance 60separating the first and second detector element axes 56 and 58 issubstantially equal to the spaced-distance 50 between the first andsecond light emitting element axes 46 and 48 (FIG. 4).

In addition, the first detector element 52 is generally aligned with thefirst track 18 of the code strip 12 so that the first detector element52 detects the indicia 20 comprising the first track 18 of code strip12. The second detector element 54 is generally aligned with the secondtrack 22 of code strip 12 so that the second detector element 54 detectsthe indicia 24 comprising the second track 22 of code strip 12.

The staggered spacing of the various detector elements (e.g., 52 and 54)comprising the detector assembly 32, that is to say, the fact that thesecond detector element 54 is located the spaced-distance 60 along thedisplacement path 26 from the first detector element 52, means that theindicia 24 comprising the second track 22 of the code strip 12 should bedisplaced or off-set by substantially the same distance, i.e., thespaced-distance 60.

The optical encoder 10 may also be provided with an aperture plate 62.The aperture plate 62 is positioned between the detector assembly 32 andthe code strip 12 in the manner best seen in FIG. 3. The aperture plate62 defines at least a first aperture 64 that is generally aligned withthe first detector element 52 and a second aperture 66 that is generallyaligned with the second detector element 40. The aperture plate 62 maybe provided with additional apertures, as will be described in greaterdetail below. In addition, and as will also be described in greaterdetail below, the aperture plate 62 may in some cases provide forincreased resolution of the optical encoder 10.

One useful feature of the optical encoder 10 is that it is readilyscalable, thus allowing it to be easily adapted to a wide range ofapplications. That is, the same basic design can be easily modified byeither increasing or decreasing the number of individual light emittingelements 37 and detector elements 51 to accommodate wider or narrowercode strips. Another useful feature of the optical encoder 10 is thatthe staggered arrangement of the light emitting elements 37 allows theoverall length 49 of the light source assembly 30 to be reduced overwhat would otherwise be the case if the staggered spacing were not used.In addition, the use of separate or staggered collimating lenses 41 foreach light emitting element 37 allows the overall thickness of the readhead 28 to be reduced over what would be otherwise required if a singlecollimating lens were used for all of the light emitting elements 37.The aperture plate 62 provides for increased sensitivity by limiting theamount of stray light that is allowed to reach the various detectorelements 51. The aperture plate 62 may also provide for increasedresolution of the optical encoder.

Having briefly described one embodiment 10 of an optical encoder, thisand other embodiments will now be described in greater detail. However,before proceeding, it is noted that the optical encoder 10 may beprovided with any of a wide range of separate light emitting elements 41and detector elements 51, depending on its application. In addition, thenumber of tracks 17 provided on the code strip 12, as well as the numberand spacing of the indicia 19 that may be provided on each track mayalso vary depending on the requirements of the particular application aswell as the desired resolution. Similarly, the optical encoder is notlimited to use in linear applications and could be readily adapted foruse in rotary applications, as would become apparent to persons havingordinary skill in the art after having become familiar with theteachings provided herein.

Referring back now to FIGS. 1 and 2, one embodiment of an opticalencoder 10 may be used to determine an absolute position of a code strip12 relative to a read head 28 of the optical encoder 10. In theembodiment shown and described herein, the read head 28 moves along adisplacement path 26 with respect to the code strip 12, which remainsstationary. Alternatively, the code strip 12 could be moveable, with theread head 28 remaining stationary. Likewise, the code strip 12 need notcomprise a generally rectangularly shaped element, but could insteadcomprise a disk-like or annular member for use in rotary applications.

In order to provide absolute position sensing, the code strip 12 isprovided with a plurality of tracks 17 (e.g., a first track 18 and asecond track 22) having indicia 19 (e.g., first set of indicia 20 andsecond set of indicia 24) provided thereon. The indicia 19 aredetectable by the detector assembly 32 in order to allow the detectorassembly 32 to detect movement of the code strip 12. Commonly usedindicia 19 include, but are not limited to, alternating regions that aresubstantially transparent and substantially opaque to the light producedby the light source assembly 30. Adjacent tracks 17 are provided withdiffering indicia to allow the absolute position of the code strip 12 tobe determined relative to the read head 28. For example, in oneembodiment each successive track 17 is provided with twice the number ofindicia (e.g., substantially transparent and substantially opaqueregions), thereby allowing each position along the code strip 12 to havea unique “code” associated therewith.

One feature of the code strip 12 that is unique relates to those tracks17 that correspond to the detector elements 51 (e.g., second detectorelement 54) that are located at the off-set or displaced position alongthe displacement path 26, such as those detector elements 51 that arearranged along the second detector element axis 58. As mentioned, theindicia 19 (e.g., indicia 24) of those tracks corresponding to theoff-set detectors (e.g., second track 22) should be off-set by the samespaced-distance (e.g., spaced-distance 60) separating the detectorelements 51.

Referring now to FIGS. 4 and 5, the light source 30 may comprise aplurality of individual light emitting elements 37, such as a firstlight emitting element 38 and a second light emitting element 40. Eachof the plurality of light emitting elements 37 is provided with acorresponding collimating lens 41, such as first collimating lens 42 andsecond collimating lens 44. The first light emitting element 38 ispositioned along a first light emitting element axis 46, whereas thesecond light emitting element 40 is positioned along a second lightemitting element axis 48. In the embodiment shown and described herein,the light source assembly 30 is provided with additional light emittingelements 37 positioned along the first and second light emitting elementaxes 46 and 48 in the manner best seen in FIG. 5. However, becausepersons having ordinary skill in the art would be able to readilyprovide such additional light emitting elements 37 after having becomefamiliar with the teachings provided herein, the additional lightemitting elements 37 that may be utilized will not be described infurther detail herein.

As was briefly mentioned earlier, the first and second light emittingelement axes 46 and 48 are separated by a spaced-distance 50 along thedirection of the displacement path 26. The staggered spacing of thefirst and second light emitting elements 38 and 40 allows the overalllength 49 of the light source assembly 30 to be reduced compared to whatwould otherwise be the case if the individual light emitting elements 37were aligned along a common axis. The magnitude of the reduced overalllength 49 will be particularly significant in the case where individualcollimating lenses 41 are used for each individual light emittingelement 37.

As was also mentioned earlier, it is noted that the optical encoder 10is not limited to use with two light emitting elements 37, such as firstlight emitting element 38 and second light emitting element 40, butinstead could comprise any number of light emitting elements 37. Forexample, in the embodiment illustrated in FIGS. 4 and 5, the lightsource assembly 30 comprises a total of four light emitting elements 37,with two light emitting elements 37 arranged along the first lightemitting element axis 46 and two light emitting elements 37 arrangedalong the second light emitting element axis 48. As mentioned, theplurality of light emitting elements 37 are staggered so as to minimizethe overall length 49 of the light source assembly 30.

The light emitting elements 37, e.g., first and second light emittingelements 38 and 40, may comprise any of a wide range of light emittingdevices that are now known in the art or that may be developed in thefuture that are or would be suitable for the intended application.Consequently, the light emitting elements 37 should not be regarded aslimited to any particular type of light emitting element 37. However, byway of example, the plurality of light emitting elements 37 may compriselight emitting diodes.

The various light emitting elements 37 may be mounted to any of a widerange of structures, such as a printed circuit board, suitable forholding the various light emitting elements 37 at the proper positionson the first side 14 of code strip 12 in the manner described herein.Alternatively, other mounting arrangements are possible, as would becomeapparent to persons having ordinary skill in the art after having becomefamiliar with the teachings provided herein. By way of example, in oneembodiment, the various light emitting elements 37 are mounted to aprinted circuit board 68 of the type well-known in the art.

As mentioned, each light emitting element 37 (e.g., first light emittingelement 38 and second light emitting element 40) may be provided with aseparate collimating lens 41 (e.g., first lens 42 and second lens 44)for collimating the light produced by the light emitting elements 37.The collimating lenses 41 may comprise any of a wide variety of lensshapes and may be fabricated from any of a wide variety of materials, aswould become apparent to persons having ordinary skill in the art afterhaving become familiar with the teachings provided herein. Consequently,the collimating lens 41 should not be regarded as limited to anyparticular type of collimating lens 41 fabricated from any particularmaterial. However, by way of example, in one embodiment, eachcollimating lens 41 comprises a convex collimating lens fabricated froma transparent plastic material (e.g., acrylic plastic). The collimatinglenses 41 may be provided with suitable mounting lugs or tabs and may besecured to the printed circuit board 68 by any convenient means, suchas, for example, by a suitable adhesive.

As best seen in FIG. 4, the staggered arrangement of the light emittingelements 37, such as first and second light emitting elements 38 and 40,as well as the corresponding staggered arrangement of the respectivecollimating lenses 41, such as first and second collimating lenses 42and 44, makes efficient use of space on the printed circuit board 68 andminimizes the overall length 49 of the light source assembly 30 overwhat would otherwise be the case if the light sources 37 and lenses 41were not staggered.

The detector assembly 32 is best seen in FIG. 6 and may comprise aplurality of detector elements 51, such as a first detector element 52and a second detector element 54. The first detector element 52 ispositioned along a first detector element axis 56, whereas the seconddetector element 54 is positioned along a second detector element axis58. The first and second detector element axes 56 and 58 are separatedby a spaced-distance 60 along the direction of the displacement path 26.In the embodiment shown and described herein, the spaced-distance 60separating the first and second detector element axes 56 and 58 issubstantially equal to the spaced-distance 50 between the first andsecond light emitting element axes 46 and 48 (FIG. 4). In addition, thefirst detector element 52 is generally aligned with the first track 18of the code strip 12 so that the first detector element 52 detects theindicia 20 comprising the first track 18 of code strip 12. The seconddetector element 54 is generally aligned with the second track 22 ofcode strip 12 so that the second detector element 54 detects the indicia24 comprising the second track 22 of code strip 12.

It should be noted that the optical encoder 10 is not limited to usewith two detector elements 51, such as first detector element 52 andsecond detector element 54, but instead could comprise any number ofdetector elements 51. For example, in the embodiment illustrated in FIG.6, the detector assembly 32 comprises a total of eleven (11) detectorelements 51, with six (6) detector elements arranged along the firstdetector element axis 56 and with five (5) detector elements 51 arrangedalong the second detector element axis 58. The use of eleven (11)individual detector elements 51 allows a ten track code strip 12 to beused, with one detector element 51 per track 17. The remaining (11^(th))detector element 51 is used to measure the intensity or light output ofthe light source assembly 30. If the intensity of the light sourceassembly 30 is too high or too low, a compensation system (not shown)may be used to adjust the electrical power provided to the light sourceassembly 30, thereby maintaining the light output within acceptablelimits. A ten track code strip 12 will provide a resolution of 2¹⁰ or1024 discrete positions. Of course, a greater or lesser number ofdetector elements 51 and code strip tracks 17 could be used depending onthe requirements of the particular application. Examples of requirementsthat would indicate the use of a code strip having a greater or lessernumber of tracks 17 include, but are not limited to, the desiredresolution as well as the range of motion that is desired to be encoded.

The staggered spacing of the various detector elements 51 (e.g., 52 and54) comprising the detector assembly 32, i.e., the fact that the seconddetector element axis 58 is located the spaced-distance 60 along thedisplacement path 26 from the first detector element axis 56, means thatthe indicia 24 comprising the second track 22 of the code strip 12should be displaced or off-set by substantially the same distance, i.e.,the spaced-distance 60.

The detector elements 51, e.g., first and second detector elements 52and 54, may comprise any of a wide range of light detecting devices thatare now known in the art or that may be developed in the future that areor would be suitable for the intended application. Consequently, thelight detecting element 51 should not be regarded as limited to anyparticular type of light detecting element 51. However, by way ofexample, the plurality of light detecting elements 51 may comprisephoto-transistors.

The various detector elements 51 may be mounted to any of a wide varietyof structures, such as printed circuit boards, suitable for holding thevarious detector elements 51 at the proper positions on the second side16 of code strip 12 in the manner described herein. Alternatively, othermounting arrangements are possible, as would become apparent to personshaving ordinary skill in the art after having become familiar with theteachings provided herein. By way of example, in one embodiment, thevarious light detecting elements 51 are mounted to a printed circuitboard 70.

The optical encoder 10 may also be provided with an aperture plate 62.The aperture plate 62 defines at least one aperture for each of thedetector element axes (e.g., first detector element axis 56 and seconddetector element axis 58) utilized on the detector assembly 32. In theembodiment shown and described herein, the aperture plate 62 defines atleast a first aperture 64 that is substantially aligned with the firstdetector element 52 on the first detector element axis 56 and a secondaperture 66 that is substantially aligned with the second detectorelement 54 on the second detector element axis 58. Additional aperturesmay be provided for each grouping of detector elements 51 that may beprovided on the detector assembly 32. Generally speaking, it will bedesirable to form the first and second apertures 64 and 66 as elongatedslits in order to minimize the chances that stray light will reach thedetector elements 51.

Depending on the particular application, the aperture plate 62 may alsobe used to increase the resolution of the optical encoder system overwhat would otherwise be possible without the aperture plate 62. Forexample, if the spacings between the various indicia 19 provided on thetracks 17 of the code strip 12 are smaller than the size of thecorresponding detector element 51, then the detector element 51 would beincapable of resolving the spacing between the indicia 19. That is, thedetector 51 would not be capable of isolating which set of indicia waspositioned directly in line with the detector element 51. In order toavoid this problem, the aperture plate 62 may be provided with anaperture having a size (i.e., width) that is substantially equal to thewidth of the indicia 19 on the code strip 12. The aperture would thenprevent light from other indicia 19 from reaching the detector element51, thereby allowing the detector element 51 to sense only the desiredportion of the code strip 12. Stated another way, the detector element51 will then be able to detect a single indicia 19 on the code strip 12,notwithstanding the fact that the size (i.e., width) of the detectorelement 51 exceeds the size (i.e., width) of the indicia 19 on the codestrip 12.

The housing 34 may be configured to receive the light source assembly30, the detector assembly 32, as well as the aperture plate 62.Referring now to FIGS. 2, 7 a, 7 b, and 8, the housing 34 may comprise adetector plate portion 72 and an emitter plate portion 74. The detectorplate portion 72 is configured to receive the detector assembly 32 aswell as the aperture plate 62. The emitter plate portion 74 isconfigured to receive the light source assembly 30. The emitter plateportion 74 is securable to the detector plate portion 72 and holds orpositions the light source assembly 30 so that it is generally alignedwith the detector assembly 32. The housing 34 also positions the lightsource assembly 30 and detector assembly 32 so that a space 36 isdefined therebetween suitable for receiving the code strip 12.

The component spacings provided by the housing 34 are not particularlycritical, and any of a wide range of spacing may be used depending onthe particular application. Consequently, the housing 34 should not beregarded as limited to a housing providing any particular spacingbetween the optical encoder's various components. However, by way ofexample, in the embodiment shown and described herein, the space 36defined between the light source assembly 30 and the detector assembly32 is about 6.25 mm; the code strip 12 is positioned about 1.9 mm fromprinted circuit board 70 of the detector assembly 32; and the apertureplate 62 is positioned about 1.23 mm from the printed circuit board 70of the detector assembly 32. See FIG. 3.

With reference now to FIGS. 7 a, 7 b, and 8, the detector plate portion72 of housing 34 may comprise a plurality of bearing surfaces 76provided thereon. The bearing surfaces 76 engage corresponding first andsecond edge portions 78, 80 of code strip 12. The bearing surfaces 76help to position the code strip 12 an optimal distance from the detectorassembly 32 and aperture plate 62, as well as to minimize the likelihoodthat the code strip 12 will contact either the light source assembly 30,the detector assembly 32 or aperture plate 62.

The bearing surfaces 76 may comprise any of a wide range of shapes andconfigurations, as would become apparent to persons having ordinaryskill in the art after having become familiar with the teachingsprovided herein. Consequently, the bearing surfaces 76 should not beregarded as limited to bearing surfaces 76 having any particular shapesor configurations. However, by way of example, in one embodiment, eachbearing surface 76 comprises a generally semi-cylindrical surface.

The various components (e.g., detector plate portion 72 and emitterplate portion 74) comprising the housing 34 may be fabricated from anyof a wide range of materials that would be suitable for the intendedapplication. By way of example, in one embodiment, the detector plateportion 72 and emitter plate portion 74 are molded from a polycarbonateplastic material. Alternatively, other materials could also be used.

1-9. (canceled)
 10. An optical encoder for determining a position of acode strip with respect to said optical encoder, the code strip havingindicia provided thereon, comprising: a light source; a detectorpositioned in spaced-apart relation from said light source so that aspace is defined between said light source and said detector, said spacebeing adapted to receive the code strip and allow the code strip to bemoved along a displacement path with respect to said optical encoder;and an aperture plate positioned adjacent said detector so that saidaperture plate is between said detector and the code strip when the codestrip is received within the space defined between said light source andsaid detector, said aperture plate defining an aperture therein that issubstantially aligned with said detector so that said detector detectsindicia on the code strip.
 11. The optical encoder of claim 10, whereinsaid light source comprises a plurality of light emitting elements and acollimating lens positioned adjacent each of said plurality of lightemitting elements.
 12. The optical encoder of claim 10, furthercomprising a housing, said housing being adapted to receive saiddetector and said light source in generally parallel, spaced-apartrelation, said housing defining a bearing surface, said bearing surfaceslidably engaging the code strip when the code strip is positioned inthe space defined between said detector and said light source, saidbearing surface maintaining a spacing alignment of the code strip withinthe space defined between said detector and said light source as thecode strip moves along the displacement path.
 13. The optical encoder ofclaim 10, wherein said bearing surface comprises a first bearing surfacepositioned at about a first end of said housing so that said firstbearing surface slidably engages a first side portion of the code stripand a second bearing surface positioned at about a second end of saidhousing so that said second bearing surface slidably engages a secondside portion of the code strip.
 14. The optical encoder of claim 13,wherein said first and second bearing surfaces comprise semi-cylindricalsurfaces.
 15. An optical encoder, comprising: a code strip, said codestrip having a first side, a second side, at least one substantiallytransparent area thereon, and at least one substantially opaque areathereon; a light source positioned the first side of said code strip,said light source directing light toward said code strip; a detectorpositioned on the second side of said code strip, said detector beingsubstantially aligned with said light source; and an aperture platepositioned between said detector and the second side of said code strip,said aperture plate defining a slit aperture therein, said slit aperturebeing aligned with said detector so that light passing through the atleast substantially one transparent area of said code strip reaches saiddetector.
 16. The optical encoder of claim 15, wherein said light sourcecomprises a plurality of light emitting elements.
 17. The opticalencoder of claim 16, further comprising a collimating lens positionedadjacent each of said plurality of light emitting elements.
 18. Theoptical encoder of claim 15, wherein said code strip comprises a firsttrack and a second track, said code strip moving along a displacementpath with respect to said detector, said detector comprising a firstdetector element aligned with the first track and a second detectorelement aligned with a second track, said second detector element beinglocated a spaced distance along the displacement path from the firstdetector element.
 19. The optical encoder of claim 18, wherein saidlight source comprises a first light emitting element positioned on thefirst side of said code strip that is substantially aligned with saidfirst detector element and a second light emitting element positioned onthe first side of said code strip that is substantially aligned withsaid second detector element.
 20. The optical encoder of claim 19,further comprising a first collimating lens positioned adjacent saidfirst light emitting element and a second collimating lens positionedadjacent said second light emitting element.
 21. The optical encoder ofclaim 20, wherein said detector comprises a printed circuit board andwherein said first detector element is mounted to said printed circuitboard on a first detector element axis, said first detector element axisbeing substantially transverse to the displacement path and wherein saidsecond detector element is mounted to said printed circuit board on asecond detector element axis, said second detector element axis beingsubstantially transverse to the displacement path, said second detectorelement axis being separated from said first axis by the spaced distancealong the displacement path.
 22. The optical encoder of claim 20,wherein said light source comprises a printed circuit board and whereinsaid first light emitting element is mounted to said printed circuitboard on a first light emitting element axis, said first light emittingelement axis being substantially transverse to the displacement path,and wherein said second light emitting element is mounted to saidprinted circuit board on a second light emitting element axis, saidsecond light emitting element axis being substantially transverse to thedisplacement path, said second light emitting element axis beingseparated from said first light emitting element axis by the spaceddistance along the displacement path.
 23. The optical encoder of claim15, further comprising a housing, said housing being adapted to receivesaid detector and said light source in generally parallel, spaced-apartrelation, said housing defining a bearing surface, said bearing surfaceslidably engaging said code strip when said code strip is positioned ina space defined between said detector and said light source, saidbearing surface maintaining a spacing alignment of said code stripwithin the space defined between said detector and said light source assaid code strip moves along the displacement path.
 24. The opticalencoder of claim 23, wherein said bearing surface comprises a firstbearing surface positioned at about a first end of said housing so thatsaid first bearing surface slidably engages a first edge portion of saidcode strip and a second bearing surface positioned at about a second endof said housing so that said second bearing surface slidably engages asecond edge portion of said code strip.
 25. The optical encoder of claim24, wherein said first and second bearing surfaces comprisesemi-cylindrical surfaces.
 26. An optical encoder for determining aposition of a code strip with respect to said optical encoder, the codestrip having alternating areas that are substantially transparent andsubstantially opaque, comprising: a light source; a detector positionedin spaced-apart relation from said light source so that a space isdefined between said light source and said detector, said space beingsized to receive the code strip and allow the code strip to be movealong a displacement path with respect to said optical encoder, saiddetector having a detection area having a width that is greater than awidth of the substantially transparent areas on the code strip; and anaperture plate defining an aperture therein, said aperture having awidth that is substantially equal to the width of the substantiallytransparent areas on the code strip, said aperture plate beingpositioned adjacent said detector so that said aperture plate is betweensaid detector and the code strip when the code strip is received in thespace defined between said light source and said detector.